High-frequency dielectric heating in a resonant chamber



March 14, 1950 E. C. HANSON ET AL HIGH-FREQUENCY DIELECTRIC HEATING IN A RESONANT CHAMBER Filed June 1, 1946 HIGH FfiEQUE/YC Y SUPPLY SOURCE .lhventors'. Earl cJ-lanson, Donald E.Watts,

by WWW Their Attorney.

Patented Mar. 14, 1950 UNITED STATES PATENT OFFICE HIGH-FREQUENCY DIELECTRIC HEATING IN A RESONANCE CHAMBER Application June 1, 18 i6, Serial No. 673,724

3 Claims. 1

Our invention relates to high frequency electric heating apparatus utilizing a chamber in which oscillating electromagnetic fields are produced, more particularly to apparatus for heating non-metallic or dielectric materials by electromagnetic fields and has for its object a simple, reliable and eflic'ient apparatus for heating 3J1? ticles and for heating material in the form of a continuous thread or strip or articles on a con veyor.

In carrying out our invention in one form we provide a chamber which may be resonant or nonresonant and which has walls made of electrically conducting material, together with means for passing the material to be heated through the chamber. All dielectric materials, both organic and inorganic, may be heated for drying, hardening or other purposes, such as paper, fabrics, rubber, rayon, cellulose or other synthetic materials, food products, tobacco, ceramic materials, etc.

For a more complete understanding of our invention reference should be had to the accompanying drawing, Fig. l. of which "is a view in perspective of high frequency heating apparatus embodying our invention, Fig. 2 is a View in perspective of a simplified form of our invention, Fig. 3 is a View in perspective of a modified form of our invention, while Fig. 4 is a sectional view of a modified form of our invention.

Referring to Fig. 1 of the drawing we have shown our invention in one form as applied to the heating of a strip I of flexible dielectric material, such as damp paper to be dried, which is moved horizontally and continuously at a predetermined speed along a predetermined path through the heating apparatus 2 by suitable feed and winding rolls and driving means (not shown) the strip 1 passing over guide rollers 3 and 4.

The heating apparatus 2 comprises a closed rectangular container 5 formed by walls of electrically conducting material, preferably copper. and provided with a slot 6 in opposite side walls near the bottom wall 1 through which slots the strip i passes through the chamber for heating inside the chamber. Electric power at a suitable high frequency is applied from electric supply means 8, which preferably is an electronic oscil-- lation generator, by a magnetic coupling or hair pin device 9 to a wave guide iii having walls made of an electrically conducting material such as copper bymeans oi graduallytapered walls to the upper wall of the chamber 5. A'sshown the con-- allel lengths or threads.

pling device 5 is connected to the source 8 by coaxial conductors the center one of which passes through an electric insulator in the wall of the wave guide. By means of the coupling device 9 high frequency oscillating electromagnetic fields are set up in the chamber, which fields are intercepted by the strip l with the generation of heat in the strip.

In order to provide for the propagation of electric waves in the chamber and energy oscillation back and forth from electric to magnetic fields at the frequency of the supply source, at least one dimension of the wave guide and the chamber is selected greater than one half of the wave length oi the generator 3. For example, with power supplied by the generator 8 at a frequency of 2900 megacycles, Whose Wave length is 15 centimeters, at least one transverse inside dimension of the wave guide and chamber must be 7.5 centimeters or greater. The wave guide may have one transverse dimension of substantially 7.5 centimeters and a length of many times this value. The chambar 5, which may be considered also as a wave guide, will probably be larger than the wave guide in a transverse or horizontal direction as shown, although its dimensions in a vertical direction may be less than the length of the wave guide. For example, the chamber 5 may be three feet square transversely and one foot high. The coupling device 9 may be located one-half wave length from the upper end of the wave guide.

For effective and efficient heating of the strip l the slots are positioned a distance from the bottom wall l where rapid and eflicient heating will be obtained, such as approximately one-fourth wave length, i. e. 3.75 centimeters for a 2000 megacycle supply source. The strip is supported the chamber on a plate I! made of low loss material as compared with the material being heated, such as glass, secured to the side walls of chamber. If desired, separate sheets of material or other objects may be heated by placing them on "the low loss plate I I.

Metal plates i2 and B3 are provided at the slot 6 respectively above and below the strip. These plates are secured to the container 5 and serve as a capacitor choke to prevent energy radiation from the chamber.

It will be understood that the strip i is heated by heat generated in the strip itself by the electromagnetic fields in the chamber 5.

The strip i may consist of a plurality of par- Latex in strip or cordform may be passed through the chamber and heated therein to vulcanize it.

In Fig. 3 we have shown our invention as applied to the heating of dielectric articles M on a conveyor belt l5 moved continuously by suitable means (not shown) over supporting rollers 16 made of electrically insulating material having a relatively low power factor. The belt is also made of a dielectric material having a low power factor such as dry cotton fabric.

In this application of our invention the resonant chamber 5 is open at one side, as shown the bottom side and suitably supported just above the conveyor belt. A bottom wall plate l8 for the chamber is provided below the rollers IS, the spacing of the plate l8 from the belt I5 and articles to be heated being such as to provide rapid and efiicient heating preferably a distance of at least substantially one-fourth of the wave length of the supply source. As shown, the plate I8 is provided with an electric ground connection IQ for safety reasons. This form of our invention is adapted for the heating of coffee beans for roasting purposes, the drying of tobacco and ceramic materials, and, in fact, the heating of all dielectric mate ials having a power factor high enough for effective heating by the high frequency used.

In the modified form of our invention shown in Fig. 4, we have shown our invention as anplied to the vulcanization of rubber, esp cially automobi e tires. The rubber t re 2% which is to be vulcanized is carried by a forming core 2| and enclosed between the annular mold sections 22 and 23. The core and mold sections are preferably made of a dielectric material such as low power factor glass, or other material having a low power factor as compared with rubber so that the generation of heat is confined substantially in the tire. It will be understood that the mold sections are suitably detachabh secured together so that they can be readily separated for the insertion and removal of the tire. For the generation of heat in the tire the mold and tire are placed in a container 24 such as dis closed in Fig. 1 except that the side walls are imperforate and the bottom wall 25 is removable for the insertion and removal of the mold and tire. The tire is positioned a distance from the bottom plate 25 such as to provide rapid and efficient heating.

If desired, the source of high frequency supply can be connected directly to the heating cl1amber through which the material to be heated is passed or in which the material is placed, as shown in Fig. 2, without using the wave guide of smaller cross section leading into the heating chamber as shown in Figs. 1, 3 and 4. As shown in Fig. 2, the container 23 made of eletrically conductin material such as copper is provided with slots 21 and 28 in its top and bot-- tom walls respectively through which the material to be heated is passed through the chamber.

The high frequency supply source (not shown) is connected to the chamber by means of a coaxial line consisting of an outer cylinder 25 having its end electrically connected to the top wall of the container, while its inner conductor 30 extends downward through an aperture in the wall of the container and is electrically connected to the bottom of the container. One in side dimension of the container, as shown the side 3|, is at least as great as one-half the wave length of the supply source in order to assure the propagation of the electric waves in the chamber required for efficient heating.

Obviously, when the heating chamber is filled with material, tuning of the chamber with the supply source will be broad and a resonant condition may not exist.

While we have shown a particular embodiment of our invention, it will be understood, of course, that we do not wishto be limited thereto since many modifications may be made and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What We claim as new and desire to secure by Letters Patent of the United States, is:

1. High frequency heating apparatus comprising a high frequency oscillation generator for supplying electric power having a predetermined high frequency, walls of electrically conducting material forming a chamber open at the bottom, means connecting said generator to said chamher for producing high frequency electromagnetic fields in said chamber, said chamber having a dimension at least as great as one-half of the wave length of said predetermined high frequency, conveyor means for moving articles to be heated past the open bottom of said chamber, and a plate made of electrically conducting material below said conveyor means forming a bottom wall for said chamber, said plate being a conveyor means for moving articles to be heated past the open bottom of said chamber, a plurality of rollers supporting said conveyor means, and a plate made of electrically conducting material below said rollers forming a bottom wall for said chamber, said bottom wall being spaced from said conveyor a distance equal to substantially one-fourth of the wave length of said high frequency to give rapid and eificient heating.

3. High frequency heating apparatus comprising a high frequency oscillation generator for supplying electric power having a predetermined high frequency, walls of electrically conducting material forming a chamber open at one side, means connecting said generator to said chamber for producing electromagnetic fields in said chamber, said chamber having a dimension at least as great as one-half of the wave length of said predetermined high frequency, means for moving a material to be heated along a predetermined path adjacent the open side of said chamber, and a plate made of electrically conducting material on the opposite side of said path.

from said chamber forming a wall for the open side of said chamber, said plate being spaced from said path a distance of at least substantially on fourth of the wave length of said high frequency.

EARL C. HANSON. DONALD E. WATTS.

(References on following page) REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Darrah May 26, 1936 Stephen Feb. 11, 1941 King Feb. 18, 1941 Mouromtseff Oct. 14, 1941 Hull Feb. 15, 1944 Hansell Dec. 5, 1944 6 Number Name Date 2,370,161 Hansen Feb. 27, 1945 2,400,777 Okress May 21, 1946 2,433,067 Russell Dec. 23, 1947 OTHER REFERENCES 

